Key Findings From LTPP Analysis 2000-2003

INITIAL ROUGHNESS

Pavement roughness greatly affects ride qualilty, safety, and vehicle operating costs. The following are
key findings from several LTPP studies to enhance understanding of how and why roughness occurs in pavements.

In the SPS-3 experiment (maintenance of flexible pavements),
the thin (38-mm (1.5-inch)) AC overlay treatment has
a small but significant effect in initial reduction of roughness.
It is the only one of the four treatments studied (thin AC
overlays, chip seals, slurry seals, and crack seals) to have a
significant effect on long-term pavement roughness.

Report No. FHWA-RD-00-029
Based on 8 years of data collected in the SPS-5 experiment
(rehabilitation of AC pavements) in the United States and
Canada, the long-term control of roughness generally can
be attained with thin (51­mm (2­inch)) and thick (127­mm (5­
inch)) AC overlays. However, the success of each project
depends on various factors such as surface preparation,
traffic loads, climatic regions, and pavement conditions
before the overlay is placed.

In the GPS-1 experiment (AC pavements on granular base),
the strongest relationships between the rate of increase of
IRI over time and an evaluated parameter exist for the following
parameters: percentage of base material passing
No. 200 sieve, freezing index, and plasticity index of subgrade.
Higher parameter values induce higher rates of IRI
increase.

In the GPS-2 experiment (AC pavements on stabilized
base), those sections built over asphalt-treated bases
with high AC void ratios (percentage of air voids per unit
AC volume) have a higher rate of increase of IRI. Also,
those sections built over cement-treated bases in
warmer climates have a higher rate of increase of IRI.

Among the doweled pavements in the GPS-3 experiment
(JPCP), those pavements with dowels have less joint faulting,
which results in lower IRI values than those pavements
without dowels. Higher IRI values are associated with a high
number of wet days.

In the GPS-5 experiment (CRCP), 90 percent of CRCP sections
are located in the wet climatic region (freeze and nofreeze).
In the region, higher levels of roughness are associated with those sections with higher PCC elastic
moduli and higher ratios between PCC elastic moduli and
tensile strength.

In the GPS-6 experiment (AC overlay of AC pavements),
the IRI rate of increase on overlaid pavements is related to
the IRI prior to overlay.

In the GPS-7 (AC overlay of PCC pavements) experiment,
initial results indicate that high rates of IRI increase were
observed for overlays on PCC sections that have high
PCC elastic modulus.

HMA pavements with unbound aggregate layers have
slightly more fatigue cracking and higher IRI values than
do those with asphalt-treated base layers.

Jointed plain concrete pavements constructed on coarsegrained
subgrade soils are smoother than pavements
constructed on fine-grained subgrade soils. This confirms
a similar finding from a previous study.

Placing overlays on pavements (flexible or rigid) that have
an IRI of less than 2.0 m/km (10.6 ft/mile) appear to be an
effective rehabilitation strategy in extending the life of the
pavement. However, the section should have sufficient
structural capacity to carry the anticipated traffic volume.

Report No. FHWA-RD-02-057

Using LTPP profile data, the basis for roughness computations, 54 models were developed to assist highway agencies in transitioning smoothness specification limits from
profile index (PI) (5, 2.5, and 0 mm (0.2, 0.1, 0 inches)) to IRI or to PI0.0. Depending on the current situation and an agency's need, appropriate models can be chosen from the 54 models for transition.